Police today named the victim as apprentice builder Rory Hayward.
Mr Hayward was a former John Paul II High School student, who graduated from the Tai Poutini Polytechnic pre-trades course at the end of last year.
Worksafe NZ has opened an investigation into what went wrong at the building site, at a group of baches near the mouth of the Poerua River.
Acting senior sergeant Andrew Lyes, of Greymouth, said in a statement that Mr Hayward was one of four workers for a building firm at the site yesterday to repile a residence.
However, police this morning were unable to clarify if the building had collapsed.
Emergency services were called to the site, past the road end at Oneone, west of Hari Hari township, shortly after 2.30pm.
Due to the remote location and the Poerua River running high at the time of the accident, emergency services had difficulty accessing the site.
Acting sergeant Jayne Bretherton said the bach which Mr Hayward had been working on was usually reached from the end of Oneone Road and down towards the mouth via the riverbed on the north bank.
However, with the river running high it took time to gain access and a jetboat had to be deployed to reach the accident.
"It's really tragic," Ms Bretherton said.
After completing a pre-apprenticeship course at the polytechnic last year, Mr Hayward began work for a Greymouth-based building company only six weeks ago.
John Paul II High School principal Kieran Stone said the school was in shock with news of the death of a young man who was "highly thought of".
The school's thoughts and prayers were with Rory's family and friends in the coming days, Mr Stone said.
"It's an absolute tragedy."
The school intended to make an announcement this afternoon during the end of term assembly.
